How much can I heighten My GPA?

I have kindof slacked off as a freshman(This year) and was wondering if there was anything I could do to get my GPA up around a 3.8-3.9. I am pretty smart, so I could do Advanced stuff. My GPA currently is 3.5. I am pretty sure I could do a 3.9 avearge for the rest of the years, and may also be able to take some AP. I am also in an advanced math class(one level up). Also I was wondering how the Admissions are weigted at colleges, like what do they take into account and how much weight they put on things. The only reason I'm asking this is because I want to be able to get into U of M Ann Arbor.